The present invention relates to a metallic mold for an optical element, which is manufactured by using the amorphous alloy having a supercooled liquid state, and to the optical element.
According to the manufacturing method of a molding die of a plastic optical element which is conventionally generally conducted, a blank (primary product) is manufactured by, for example, steel or stainless steel, and an alloy of an amorphous-like nickel and phosphorous is filmed in the thickness of about 100 μm by a chemical plating, which is called electroless plating, on it, and this plating layer is cutting processed by a diamond tool of an ultra-precise processing machine, and a highly accurate optical surface transfer surface for molding the optical surface of the optical element is obtained.
According to such a method of the conventional technology, because shapes of parts are created basically by the mechanical processing, the part accuracy is easily increased to near the movement accuracy of the processing machine. However, on the other hand, problems are generated in which: the mechanical processing and chemical plating processing are mixed in the manufacturing process and it is vexatiously complicated, and a long period of delivery is necessary; the manufacturing of blank (primary processed goods) is necessary, considering the thickness of plating layer; the plating processing is not always stable, and due to a deviation of composition of the blank or a soiled condition, the adhesion strength of the plating layer is dispersed, or a pinhole-like defect which is called a pit, is generated; and because the creation of the optical surface transfer surface is necessary in the thickness of plating layer, there is a case where there is not any margin in the plating thickness when the optical surface transfer surface is re-worked, and the processing can not be conducted.
Further, according to the conventional technology, it is necessary that the optical surface transfer surface is largely cutting processed by a diamond tool, however, in such a case, the influence such as a condition of a cutting edge of the tool or processing condition, a change of processing environmental temperature is exerted, and there is also a problem that the shape of the optical surface transfer surface finished after cutting processing, is delicately dispersed. This processing dispersion of the optical surface transfer surface is due to the poorness of the machinability of the raw material. In general, the optical surface shape error of about 100 nm is generated, and even in the case where processing is conducted very carefully, the shape error of about 50 nm is remained. This is a limit of processing accuracy when a large amount of optical surface transfer surfaces of the same shape are created.
Further, recently, an optical element in which a ring-shaped zone diffraction groove (ring-shaped diffractive zone) is provided on the optical surface and by which the chromatic aberration is efficiently corrected, is put into practical use in the optical information recording field, and a large amount of optical elements are manufactured. As the optical material, plastic or glass is used, however, in the infrared optical system, a crystal material such as ZnSe is also used. Such an optical element can be produced in a large amount and effectively by molding, however, it is a very important problem, at the time of molding, how high-accurately and effectively minute diffraction grooves on the optical surface of the optical element are manufactured by the metallic mold for optical element.
For example, when minute patterns having the optical function such as the diffraction grooves are created by the diamond cutting on the optical surface transfer surface of the metallic mold for optical element, the sharpness of the cutting edge controls the exactness of the diffraction groove shape, and when it is transferred as the optical surface of the optical element, the diffraction efficiency is largely influenced.
Accordingly, for the purpose that the diffraction efficiency of the ring-shaped diffractive zone is not lowered, it is necessary that the dimension of the cutting edge is made sufficiently small, in such a case, because, on the small cutting edge portion, the cutting resistance is concentrically imposed, it is necessary that the incised amount is decreased, and the number of time of processing is increased until the entire optical surface is uniformly cut and removed. Further, it is necessary that the feed speed of the tool is made slow in order also to prevent the deterioration of the surface roughness of the optical surface by the cutter mark of the small cutting edge, and the optical surface transfer surface processing time per one time also becomes long. As a result, in the cutting processing of the metallic mold for molding the optical element having the diffraction groove, because the cutting length is increased, wearing of the cutting edge of the tool is increased, and the tool change becomes often. That is, when the optical surface transfer surface having minute shape is processed by the conventional diamond cutting, because the life of the tool is very shortened and a time period for processing one optical surface transfer surface is also increased, it is necessary that the tool is changed often, therefore, the processing efficiency is very much lowered, and the productivity of the metallic mold for optical element is lowered, resulting in the rapid increase of the cost. Therefore, particularly, when the optical surface transfer surface having minute shapes on the surface is finished by the diamond cutting, the metallic mold manufacturing method which does not include the electroless nickel plating process, and is simple, and whose delivery date is short, is desired.
In addition to that, recently, it is tried that the minute structure, which is several times smaller or less than the wavelength of the light source to be used, is provided on the optical surface and a new optical function is added to the optical element. For example, the ordinary light converging function by the refraction of the molded lens and a positive dispersion generated as the side reaction at the time are cancelled by using a large negative dispersion by the diffraction obtained when the diffraction groove is provided on the surface of the aspheric surface optical surface, and a method in which the achromatic function, which is originally impossible only by the refraction, is added to a single lens optical element, is put in practice in the objective lens for pick-up apparatus for optical disk, which is DVD/CD compatible. This uses the diffraction action by the diffraction groove whose dimension is several-ten times larger than the wavelength of the light transmitting the optical element, and an area in which the diffraction action by the structure sufficiently larger than the wavelength is managed in this manner is called a scalar area.
On the one hand, it is well known when, in a minute interval which is one several-th of the wavelength of the light transmitting the optical element, protrusions of the conical shape are formed under the crowded condition on the surface of the optical surface, the reflection suppress function of the light can be exhibited. That is, the refractive index change on the border surface between the air and the optical surface when the light wave is incident on the optical element, is not instantly changed from 1 to the refractive index of the medium as in the conventional optical element, but, it is gently changed by the conical shape of protrusions arranged in minute interval, thereby, the reflection of the light can be suppressed. The optical surface on which protrusions like this are formed is a minute structure which is so-called a moth eye, and when the structural bodies which are minuter than the wavelength of the light are arranged in a period shorter than the wavelength, each structure does not diffract any more, and acts on the light wave so as to give an average refractive index. Such an area is generally called an equivalent refractive index area. Relating to such an equivalent refractive index area, for example, it is written in the collected papers C of the Institute of electronic information communication, Vol. J83-C No. 3 pp. 173-181, March 2000.
According to the minute structure of the equivalent refractive index area, a reflection prevention effect larger than the conventional reflection prevention coat can be obtained while the angle dependency or wavelength dependency of the reflection prevention effect is deceased. However, according to plastic molding, because the optical surface and the minute structure are simultaneously created, it is considered that a merit in the production in which the lens function and the reflection prevention function are simultaneously obtained, and the after processing that the reflection prevention coat processing is conducted after the molding as in the conventional one is not necessary, is large, and is remarked. Furthermore, when such a minute structure of an equivalent refractive index area is arranged in such a manner that it has the directionality to the optical surface, the strong optical anisotropy can be given to the optical surface. Therefore, the double refraction optical element, which is conventionally manufactured by cutting the crystal such as quartz crystal, can be obtained by molding. Further, when it is combined with a refraction or reflection optical element, a new optical function can be added to it. The optical anisotropy in this case is called a structural double refraction.
There is a resonance area in which the diffraction efficiency is rapidly changed by a slight difference of the incident condition, between the above-described scalar area and the equivalent refractive index area. For example, when the groove width of the ring-shaped diffractive zone is brought to narrow, a phenomenon (anomary) that the diffraction efficiency is rapidly decreased or increased at about several-times of the wavelength is generated. When the characteristic of this area is used, a wave-guide mode resonance lattice filter by which only a specific wavelength is reflected, is realized by the minute structure, and the same effect as an ordinary interference filter can be realized by a smaller angle dependency.
Hereupon, when the optical element is formed by using the scalar area, equivalent refractive index area or resonance area, it is necessary to form the minute protrusions (or recesses) on the optical surface. In order to make mass-production of the optical element having such minute protrusions (or recesses), generally, it can be said appropriate that the molding is conducted by using plastic as a raw material. However, in such a case, it is necessary that the optical surface transfer surface provided with the recesses (or protrusions) corresponding to the minute protrusions (or recesses) is provided in the metallic mold for molding of the optical element.
However, relating to the protrusion (or recess) of the equivalent refraction area or resonance area as described above, because it is necessary that protrusions (or recesses) are formed at the interval of several-tens or several-hundreds nm, it is very difficult by the mechanical processing including the cutting processing.
In view of such a problem, in Patent Document 1, the manufacturing method, in which the amorphous alloy having the supercooled liquid state is adhered to the substrate, and by processing such an amorphous alloy, the optical surface transfer surface for molding the optical surface of the optical element is formed, is disclosed. Because the amorphous alloy having such a supercooled liquid state is excellent in the processing easiness, even when it is necessary that the minute structure is formed, for example, on the optical surface transfer surface, it can be easily conducted.
[Patent Document 1] Tokkai No. 2003-160343
Hereupon, in the conventional metallic mold for the optical element, in the case where a temperature range heated when the optical element is molded is not lower than 400° C., there are many cases where the super-hard material or heat resistive material such as SiC is selected as the metallic mold material. However, because the hardness of these materials are hard materials of Hv 1000-3000 in Vickers hardness, when compared to the electroless nickel plating (about Hv 500-600), which is easily cut, they are very difficult in processing, it is difficult to increase the processing shape accuracy or the mirror property of the metallic mold molded transfer surface, or the processing time period more than four times than that of the plating materials is also necessary. As one of its causes, when the super-hard material or material such as SiC is cutting•grinding processed by the diamond tool, because they are hard materials, tool is worn and shape is deformed, there is a case where the cutting amount or cutting flaw•grinding flaw•are changed in the processing, thereby, the shape accuracy or mirror property is largely influenced. Further, also from the point of crystallinity of the material, because, in the material in which SiC, super-hard material or polycrystalline substance is sintered, the grain boundary exists, the material is one which is hardly processed, in contrast to that, there is the difference that the electroless plating is the amorphous uniform film and the material which is easily cut ground.
In contrast to this, as written-in Patent Document 1, when the amorphous alloy having the super-cooling area is adhered to the substrate of the metallic mold for the optical element, the processing easiness can be secured to a certain degree. However, in the Patent Document 1, a specific adhering amount or temperature range is not regulated, and by the content written in the Patent Document 1, it is difficult that the metallic mold for the optical element is actually manufactured.